A large proportion (some estimates are as high as twenty-five percent) of the electricity generated in the United States each year goes to lighting. Accordingly, there is an ongoing need to provide lighting which is more energy-efficient. It is well-known that incandescent light bulbs are very energy-inefficient light sources—about ninety percent of the electricity they consume is released as heat rather than light. Fluorescent light bulbs are more efficient than incandescent light bulbs (by a factor of about 10) but are still less efficient as compared to solid state light emitters, such as light emitting diodes.
In addition, as compared to the normal lifetimes of solid state light emitters, e.g., light emitting diodes, incandescent light bulbs have relatively short lifetimes, i.e., typically about 750-1000 hours. In comparison, light emitting diodes, for example, have typical lifetimes between 50,000 and 70,000. Fluorescent bulbs have longer lifetimes (e.g., 10,000-20,000 hours) than incandescent lights, but provide less favorable color reproduction.
Color reproduction is typically measured using the Color Rendering Index (CRI Ra) which is a relative measure of the shift in surface color of an object when lit by a particular lamp. CRI Ra is a modified average of the measurements of how the color rendition of an illumination system compares to that of a reference radiator when illuminating eight reference colors. The CRI Ra equals 100 if the color coordinates of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the reference radiator. Daylight has a high CRI (Ra of approximately 100), with incandescent bulbs also being relatively close (Ra greater than 95), and fluorescent lighting being less accurate (typical Ra of 70-80). Certain types of specialized lighting have very low CRI Ra (e.g., mercury vapor or sodium lamps have Ra as low as about 40 or even lower).
Another issue faced by conventional light fixtures is the need to periodically replace the lighting devices (e.g., light bulbs, etc.). Such issues are particularly pronounced where access is difficult (e.g., vaulted ceilings, bridges, high buildings, traffic tunnels) and/or where change-out costs are extremely high. The typical lifetime of conventional fixtures is about 20 years, corresponding to a light-producing device usage of at least about 44,000 hours (based on usage of 6 hours per day for 20 years). Light-producing device lifetime is typically much shorter, thus creating the need for periodic change-outs.
Designs have been provided in which existing LED component packages and other electronics are assembled into a fixture. In such designs, a packaged LED is mounted to a circuit board or directly to a heat sink, the circuit board is mounted to a heat sink, and the heat sink is mounted to the fixture housing along with required drive electronics. In many cases, additional optics (secondary to the package parts) are also necessary.
The expression “light emitting diode” is used herein to refer to the basic semiconductor diode structure (i.e., the chip). The commonly recognized and commercially available “LED” that is sold (for example) in electronics stores typically represents a “packaged” device made up of a number of parts. These packaged devices typically include a semiconductor based light emitting diode such as (but not limited to) those described in U.S. Pat. Nos. 4,918,487; 5,631,190; and 5,912,477; various wire connections, and a package that encapsulates the light emitting diode.
In substituting light emitting diodes for other light sources, e.g., incandescent light bulbs, packaged LEDs have been used with conventional light fixtures, for example, fixtures which include a hollow lens and a base plate attached to the lens, the base plate having a conventional socket housing with one or more contacts which is electrically coupled to a power source. For example, LED light bulbs have been constructed which comprise an electrical circuit board, a plurality of packaged LEDs mounted to the circuit board, and a connection post attached to the circuit board and adapted to be connected to the socket housing of the light fixture, whereby the plurality of LEDs can be illuminated by the power source.
Although the development of light emitting diodes has in many ways revolutionized the lighting industry, some of the characteristics of light emitting diodes have presented challenges, some of which have not yet been fully met. For example, the emission spectrum of any particular light emitting diode is typically concentrated around a single wavelength (as dictated by the light emitting diode's composition and structure), which is desirable for some applications, but not desirable for others, (e.g., for providing lighting, such an emission spectrum provides a very low CRI Ra).
Traditional recessed light fixtures comprise a metal cylinder (“can”) mounted into the ceiling using horizontal metal struts attached to the ceiling joints. Within the cylinder, mounted on an adjustable sliding plate, is light bulb socket into which a light bulb is inserted. Typically a 60 W incandescent light bulb or a 15 W compact fluorescent bulb is used. For example, a conventional recessed light fixture is depicted in FIG. 1.
Around the annulus of the bulb and extending downward to the lower inside edge of the cylinder is a baffle or reflecting cone. A cosmetic ring is placed around the bottom edge of the cylinder and extending outward to cover the area immediately surrounding the circular cutout in the ceiling and create an attractive edging around the light output aperture.
The position (height) of the bulb can be set such that the bulb is not directly visible to the people standing in the room and the light from the fixture is directed so that it principally illuminates an area below the fixture.
These fixtures are popular because they do not create significant glare (being recessed) and highlight objects situated below them.
The “cans” are generally required to be substantially airtight around the sides and top to prevent the loss of ambient heat or cooling from the room into the ceiling cavity through the fixture. As the lamp is mounted in the can, much of the heat generated by the light source is trapped within the can, because the air heated in the can rises and is trapped within the can. Special insulation is usually required around the can within the ceiling cavity to prevent fire.
The environment inside the can is not ideal for solid-state lighting. LEDs, for example, have significant energy and lifetime benefits over incandescent and fluorescent light sources—LEDs, however, do not operate well in high temperatures. LED light sources have operating lifetimes of decades as opposed to just months or 1-2 years for the others mentioned. An LED's lifetime is significantly shortened, however, if it operates at elevated temperatures. It is generally accepted that the junction temperature of an LED should not exceed 70 degrees C. if a lifetime of 100,000 hours is desired.
Efficient individual LED light sources typically provide between 3 and 50 lumens of light per source (LED die or lamp) depending on the size of the LED die. A typical recessed downlight using a 60 W incandescent bulb provides about 500 lumens of light, so it can be seen that to provide a similar amount of light from LEDs, multiple LED light sources would be required.
To provide a similar amount of light using “small die” (typically 300 square micrometers), approximately 200 die would be required, or, alternatively using large “power” die (typically approximately 1 square millimeter) 10-20 die would be required.
For the above and other reasons, efforts have been ongoing to develop ways by which solid state light emitters can be used in place of incandescent lights, fluorescent lights and other light-generating devices in a wide variety of applications. In addition, where light emitting diodes (or other solid state light emitters) are already being used, efforts are ongoing to provide light emitting diodes (or other solid state light emitters) which are improved, e.g., with respect to energy efficiency, color rendering index (CRI Ra), contrast, efficacy (lm/W), cost and/or duration of service.